Extensive recent research into devices for the detection of inflammable or toxic gas in air has been centred on tin(IV) oxide (SnO.sub.2) based thick film or sintered powder type sensors, these are commonly referred to as SnO.sub.2 gas sensors. The mechanism of operation of such transducers relies on the conductivity changes experienced by the n-type semiconducting material when surface chemisorbed oxygen reacts with reducing gases such as carbon monoxide (CO) or methane (CH.sub.4) at elevated temperatures. For carbon monoxide the overall reactions occurring on the SnO.sub.2 surface can be written simply: EQU 2e.sup.- +O.sub.2 .fwdarw.20.sup.- ( 1) EQU O.sup.- +CO.fwdarw.CO.sub.2 +e.sup.- ( 2)
where e.sup.- represents a conduction band electron. In the absence of reducing gas (e.g. CO), electrons are removed from the semiconductor conduction band via the reduction of molecular oxygen, leading to a build-up of O- species and consequently the SnO.sub.2 becomes very resistive. When CO is introduced, it undergoes oxidation to CO.sub.2 by surface oxygen species and subsequently electrons are re-introduced into the conduction band leading to a decrease in this resistance.
The main advantages of SnO.sub.2 based sensors include high sensitivity, low cost, fast response speed and low power consumption. However, there are also significant drawbacks such as long term drift and ambient humidity and temperature effects associated with such devices. Attempts have been made to overcome this last effect by operating these devices under very accurately temperature-controlled conditions though it is important to recognise that it is the surface and not the bulk temperature which controls the response. Another problem with SnO.sub.2 based sensors is their relative lack of selectivity, since the chemisorbed oxygen (responsible for controlling surface conductivity) reacts with a wide range of reducing gases. Several approaches have been investigated with a view to enhancing specificity, including the use of filters or specific surface additives. These, along with other methods outlined briefly below have been reviewed recently by Morrison (Sensors and Actuators, 14 (1988) 19-25).
A certain degree of selectivity can be introduced by operating a SnO.sub.2 sensor at different temperatures: Firth et al (Ann. Occup. Hyg., 18 (1975) 63-68) observed that a temperature of 300.degree. C. resulted in response to the presence of CO but none to CH.sub.4, while temperatures over 600.degree. C. favoured CH.sub.4 detection. Experiments have been performed by the applicants to see if this effect can be used by operating sensors in a temperature modulated mode. While good discrimination could be obtained for CO and CH.sub.4 by operating the sensor at different temperatures, the presence of hydrogen (H.sub.2) interfered. Not only did the sensors respond to H.sub.2 at all temperatures but the response was erratic. There was also some evidence that gases were being adsorbed and then desorbed in subsequent tests (i.e. the sensors show a "memory effect").
By far the most popular method of achieving specificity is by the addition of catalysts or promoters. Researchers have reported that inclusions such as thorium dioxide confer CO selectivity, while the presence of silver improves H.sub.2 response although not removing sensitivity to CO and CH.sub.4. Such H.sub.2 sensors incorporating silver have been reported by Yamazoe et al [(J.Chem. Sec. Japan them. Lett. (1982), 1899-1902) and (Sensors and Actuators, 4 (1983) 283-289)]in report showing apparently impressive sensitivity and selectivity for hydrogen. However the sensors described have several drawbacks.
Firstly they are not as sensitive and selective for hydrogen as appears at first sight from the graphs of results since, for example, H.sub.2 concentration of 0.8% are being compared with CO concentrations of 0.02%, a forty-fold difference in concentration that dramatically effects the presentation of results.
Secondly peak hydrogen sensitivity is shifted to a temperature of .apprxeq.100.degree. C. which is very low for such sensors which are usually held at in excess of 200.degree. C. At such low temperatures as 100.degree. C. problems can arise with condensation of volatiles on the sensor, even moisture being a problem. Additionally at such low temperatures the response time for the sensors would be very high. Response times are quoted in the above mentioned reports of 20 seconds to obtain a 90% response at 200.degree. C.
Thirdly the response is not linear with concentration (see FIG. 3 of the Sensors and Actuators paper). and so complex calibration would be required.
The same authors have studied the effects of catalysts such as manganese, nickel, cobalt or copper on sensor response to various gases including CO, CH.sub.4, H.sub.2 and propane, while in most modern commercial sensors the presence of trace quantities (0.5-5% w/w) of palladium or platinum is an essential prerequisite.
There has also been described (Sensors and Actuators, 7 (1985) 89-96) the modification of tin oxide based gas sensors by inclusion of .about.15% w/w bismuth oxide (Bi.sub.2 O.sub.3) to give CO selectivity or 36% w/w aluminium silicate and 1.5% w/w palladium chloride (PdCl.sub.2) for CH.sub.4 selectivity. The CO selective sensor described was very sensitive to the amount of Bi.sub.2 O.sub.3 present. Below 15% w/w the sensor was CH.sub.4 sensitive; above 17% w/w the sensitivity to CO began to fall effectively disappearing at between 20-30% w/w.
U.K. Patent Application 2149123A describes sensors including, inter alia, Bi.sub.2 Sn.sub.2 O.sub.7 and a gas sensitive material. Although, in discussing this material, the U.K. patent application describes gas sensitivity to O.sub.2, CH.sub.4, CO, H.sub.2, C.sub.2 H.sub.4, and NH.sub.3 no indication is given as to relative sensitivities and selectivities for these gases.